In such filter devices (as, for example, described in the German printed application Nos. 1 005 493, 1 761 617 or in the German open application Nos. 1 944 438, 2 363 040), so-called precoat filters, a filter-aid agent suspended in a carrier medium is applied to a support fabric in the first working phase, the filter-aid agent depositing as a filter layer in the first compartment, while the carrier medium enters into the second compartment and exits again.
In a second working phase, the filtering phase, the medium to be filtered and contaminated with solid particles is introduced into the first compartment. The medium traverses the filter-aid layer, is conveyed into the second compartment as a pure filtrate and again removed from said second compartment. The solid particles deposit on the filter-aid layer which becomes increasingly clogged during this working phase. The third phase is the cleaning phase in which the filter layer is backwashed from the support fabric together with the contaminants and removed from the first compartment by a washing medium. So-called cartridge filters are frequently used as a filter device in which a partition of the substantially cylindrical filter chamber is obtained by filter cartridges which are parallel to the axis and closed on one side. They are inserted into a radially arranged intermediate plate. The filter cartridges--which are tube-shaped support fabrics--have a large filter surface but because of their orientation they are difficult to clean.
A further prior art filter device of this kind has been described in the German Pat. No. 965,850. It is of similar structure but no filter cartridges are provided in the filter chamber. A substantially star-shaped support fabric surrounds a central tube and separates the two compartments from each other. In addition to the afore-described openings in the two compartments, the central tube forms a further closable outlet opening from the second compartment which serves for the backwashing of the carrier medium during the precoat phase.
Beside such precoat filters, in which the cleaning should be carried out automatically, filter presses have been used. They do, in the first place, not serve for the cleaning of a medium to be filtered but, above all, for the recovery of the filter cake. Hence, they are no precoat filters as a filter-aid agent is not desired. They have a number of adjacently or superjacently arranged filter chambers which are provided with filter cloths. Walls movable under pressure, membranes in particular, are used for pressing the filter cake. Such filter presses have, for example, been described in Austrian Pat. Nos. 233,597, 256,866, 273,169, in the German open application No. 1 805 478, in the Swiss Pat. Nos. 399,422 and 490,879 or in U.S. Pat. Nos. 3,098,429 and 3,270,887. All of the afore-mentioned filter presses have to be disassembled for removing the filter cake, whereby the individual filter plates have to be spaced from one another by such a distance that the filter cake can be removed parallel to the planar filter cloth perpendicularly to the longitudinal axis.
One or two filter cloths may be arranged in one filter chamber, and one movable wall is provided for each filter cloth, the wall being adapted to be pressed against the filter cloth. The filter cloth and its movable wall may be arranged on different filter frames. It has proved advantageous, however, to provide only one filter frame for each filter chamber, whereby the filter cloth is fixed directly to the movable wall, the wall being provided with grooves or the like for the removal of the filtrate.
Finally, the German open application No. 2 821 768 (see also U.S. Pat. Nos. 4,285,821 and 4,297,211) describes a precoat filter having a filter chamber of square cross-section. The filter chamber is divided into two compartments by a partition wall, e.g. a grid, having through openings. The support fabric is fixed to the partition wall which is preferably divided into two exchangeable grid inserts, each comprising one part of the support fabric. The partition wall serves as a support for the support fabric in order to reduce as far as possible the non-uniform strain working on the filter cloth during filtration, which is due to the square cross-section. Moreover, the system of German application No. 2 821 768 employs a movable wall in each compartment for changing the volume, whereby first, the filter cake is pressed and, second, the filter chamber can be entirely discharged between two working phases so that the mixture of residues of different media is eliminated.